For starters, I would use Sinatra where you use Rack.
It takes care of a lot of unknowns for a lot of people. For instance, it makes sure you do proper redirects. Did you know that IE9 implements 302 like 307 but only for redirects from Ajax? But you can't simply use 303 as some HTTP clients don't properly implement it? Rack does not care about such things.
It also comes with built-in security, which Rack does not.
If you are comfortable with Rails and it suits your needs (that is, a monolithic model driven application), stick with it, there is nothing wrong with Rails.
It takes care of a lot of unknowns for a lot of people. For instance, it makes sure you do proper redirects. Did you know that IE9 implements 302 like 307 but only for redirects from Ajax? But you can't simply use 303 as some HTTP clients don't properly implement it? Rack does not care about such things.
It also comes with built-in security, which Rack does not.
If you are comfortable with Rails and it suits your needs (that is, a monolithic model driven application), stick with it, there is nothing wrong with Rails.